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hdevtools is a command line program powered by the GHC API, that provides services for Haskell development. hdevtools works by running a persistent process in the background, so that your Haskell modules remain in memory, instead of having to reload everything each time you change only one file. This is just like :reload in GHCi - with hdevtools you get the speed of GHCi as well as tight integration with your editor.
This is the Vim plugin that integrates Vim with hdevtools.

The Haskell mode plugins provide advanced support for Haskell development
using GHC/GHCi on Windows and Unix-like systems. The functionality is
based on Haddock-generated library indices, on GHCi's interactive
commands, or on simply activating (some of) Vim's built-in program editing
support in Haskell-relevant fashion. These plugins live side-by-side with
the pre-defined |syntax-highlighting| support for |haskell| sources, and
any other Haskell-related plugins you might want to install (see
|haskellmode-resources|).
The Haskell mode plugins consist of three filetype plugins (haskell.vim,
haskell_doc.vim, haskell_hpaste.vim), which by Vim's |filetype| detection
mechanism will be auto-loaded whenever files with the extension '.hs' are
opened, and one compiler plugin (ghc.vim) which you will need to load from
your vimrc file (see |haskellmode-settings|).

Displaying the type of sub-expressions (ghc-mod type)
Displaying error/warning messages and their locations (ghc-mod check and ghc-mod lint)
Displaying the expansion of splices (ghc-mod expand)
Completions are supported by another plugin. See neco-ghc .

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Syntastic supports Haskell and several other languages. From the github

Syntastic is a syntax checking plugin that runs files through external syntax checkers and displays any resulting errors to the user. This can be done on demand, or automatically as files are saved. If syntax errors are detected, the user is notified and is happy because they didn't have to compile their code or execute their script to find them.
At the time of this writing, syntax checking plugins exist for applescript, c, coffee, cpp, css, cucumber, cuda, docbk, erlang, eruby, fortran, gentoo_metadata, go, haml, haskell, html, javascript, json, less, lua, matlab, perl, php, puppet, python, rst, ruby, sass/scss, sh, tcl, tex, vala, xhtml, xml, xslt, yaml, zpt

Neco-ghc power by ghcmod-vim for completion of pragma, modules, functions and more.

Addition list with some missing here with screen shots of many of the above.